Fire warning device



1951 D. J. M DOUGALL ETAL 2,577,973

FIRE WARNING DEVICE Filed July 22, 1948 17VEIflur5 DONAL 0 .1,MACDOUGALL PHILLIP J. 0405 Ema 7 3w? Patented 11, 195i FIRE WARNINGnnvrcn Donald J. MacDougall, Framingham, and Phillip J. Cade, Sharon,Mass., assignors to Photoswitch, Incorporated, Cambridge, Ma'ss., acorporation of Massachusetts Application July 22', 1948, Serial No.40,104

This invention relates to fire detection and, in particular, to firealarm devices suitable for use on vehicles, such as aircraft.

The problem of fire protection in vehicles, particularly in aircraft,involves many factors not present in stationary fire alarminstallations. A fire in an airplane engine, for example, is likely toresult in total destruction of the craft unless the proper fire fightingmeasures are quickly applied. Instant operation of the fire alarm systemupon the first occurrence of flame is, therefore, imperative.Exceptional reliability is required in such a system, because, on theone hand, its failure to operate may result in loss of the airplane andthe personnel aboard, and, on the other hand, a false alarm maynecessitate a forced landing. To insure reliability, it is essential toeliminate the possibility of false operation due to vibration or toambient temperature fluctuations, and to construct the system so thatfailure or destruction of one of the detecting elements, a number ofwhich are ordinarily located in the danger region, neither affects theoperation of the remainder of the system nor results in a false alarm.Considerations of weight and space are also of primary importance.

The fire alarm system used heretofore in aircraft have depended on Ithermally operated switching devices or fusible elements for flamedetection. Since devices which depend on secondary heating effects fortheir operation are subject to inherent delay and are influenced bylocal conditions of heat transmission, it has been found that radiationsensitive devices, such as the photoelectric cell, or the lead sulphidesemiconductor, which are excited directly by radiation from the flame,offer a far more satisfactory solution to the essentiallythree-dimensional problem of fire detection. A radiation sensitivedevice of this type is used in conjunction with an amplifier circuitwhich operates an electrical control or indicator in response toexcitation of the device. For use in aircraft, such a circuit, in addition to providing the necessary operational characteristics, must berelatively simple, and stable in operation. If a number of detectingelements are connected to a common amplifier, it may be necessary toadjust the sensitivity of the detecting channels independently, to allowfor different ambient light levels at the various detecting stations.

The object of this invention is to provide an electrical fire alarmsystem, employing radiation sensitive detecting elements, which is fastand reliable in operation, which is insensitive to vibration, ambienttemperature, and line voltage 5 Claims. (or. 171- 311) r 2 fluctuations,which is unaffected by damage resulting in either short or open circuitin a detecting element. or its connecting cable, which allows forindependent sensitivity adjustment of the various detecting channels tocompensate for difference in ambient light conditions, and whichrequires a minimum number of electrical components.

These and other objects, advantages and features will be apparent fromthe description with reference to the drawings, in which:

Fig. 1 is an electrical diagram of the circuit for a complete fireprotection system according to the invention; and

Fig. 2 is a simplified diagram illustrating one of the detectingchannels of Fig. 1.

Referring to Fig. 1, a number of phototubes 3 may be located in thespace to be supervised, for example, in the engine nacelle of theaircraft, so as to receive radiation from fiame arising anywhere withina selected region. Two or more phototubes may be connected in paralleland located remotely from the remaining circuit components. Eachphototube shown in the drawing is connected by a cable It to the grid 8of an amplifier tube 6 which is employed as a cathode follower. Thecable It! is surrounded by conductive shield II which is connected inseries with resistance l2. The cable I0 and resistances i2 and [3together form an input network to grid 8 similar to the networkcomprising resistance l2, capacitance I4 and resistance-l3 in Fig. 2.The system is supplied with alternating current at terminals 25 and 26,across which is connected primary 22 of transformer 2|.

In Fig. 1 the cathode follower tubes 6 are shown connected in parallelin pairs, the upper pair being connected to the cathode followerresistor l5, and the lower pair to cathode follower resistor IS. Adirect plate voltage is supplied to tubes 6 by a conventional directcurrent power supply, generally indicated as H, energized by secondary24 and comprising rectifier tube 29, and a filter network consisting ofcondensers 32 and 33, and resistors 30 and 3|. The direct current outputis applied across load resistances 34 and 35 which are connected in theplate circuits of tubes 6.

A tube 40, which is preferably of the hot cathode gas discharge type, issupplied with an alternating plate potential by secondary 23. Thecontrol grid 42 of tube 40 is connected through limiting resistance 39to one end of resistance 31, which acts as an input resistance to tube40. Aninitial negative bias is applied to the grid 42 by the section ofresistance 25 between its lower end and tap l6.

The operation of the circuit can best be understood with reference toFig. 2; which illustrates a single detecting channel. In Fig. 2, for thepurpose of simplifying the explanation of the circuit,

-power supply ll of Fig. 1 may be considered as replaced by a battery26, and the section of resistance 36 between its lower end and tap 36,which provides the initial negative bias on the grid of tube 46, asreplaced by a variable bias battery 45. In this circuit when thephototubes 3 are dark, no current flows across the input networkconsisting of resistances l2 and I2, and capacitance l4 which representsthe cable capacitance between cables lil and shields ii, and the bias ongrid 6 is entirely supplied by cathode follower resistors l5 and 21. Thesmall initial current which flows through tube 6 under this condition isstabilized by the presence of the cathode follower resistors which applya compensating bias change to grid 6 when changes in the plate currentof tube 6 occur. The adjustable tap I8 is so positioned that thepositive bias applied by resistance 21 to grid 42 is not sufllcient toovercome the bias applied by battery 45. Tube 40 is non-conductive andsignal lamp 44 or other load device, which is connected in the outputcircuit of tube 40 is, therefore, not energized.

If the phototubes 3 are subjected to ambient light, current flowsthrough them on alternate half cycles charging capacitanccs i4 so as toapply a positive bias to grid 8. The initial current, through tube 6under these conditions, is larger than when the phototubes are dark, butremains stable because of the cathode follower connection. Tap is can beadjusted so as simultaneously to increase the load resistance in the'plate circuit of tube 6, thereby cutting down the plate current, and toreduce the proportion of the output applied as bias to grid 42, so thattube 40 can be maintained in the non-conductive state with a reasonablenormal level of illumination at the phototubes. If the excitation of oneof the phototubes is suddenly increased, as by the outbreak of flamewithin the supervised area, the charge on the associated capacitance I4is con? siderably increased, raising the potential of grid 6. The cablecapacitance for normal lengths is relatively small so that the fullcharge is built up within less than a second. The D. C. signal which isapplied across load resistance 31 then raises the potential of grid 42sufliciently to make tube 40 conductive on alternate half cycles andsignal lamp 44 is illuminated. The lamp, therefore, gives an indicationof the presence of flame in the space supervised by either or both ofthe phototubes 3. If one of the phototubes should become open-circuited,that detecting branch drops out of action, but the other phototube mayimpress a signal on the system in the normal manner upon occurrence offlame. If the phototubes are short-circuited or bridged by a leakageresistance which is low in comparison to the dark impedance of aphototube, alternating current flows across capacitance i4 so that nocharge is built up. The alternating potential across resistance l3 givesrise to an intermittent current in the plate circuit of tube 6. As willbe seen from the arrangement of secondaries 23 and 24, however, theresulting potential across load resistance 31 is 180 out of phase withthe plate voltage of tube 40 and tube 40 therefore remainsnon-conductive. A short circuit across a capacitance l4, which isequivalent to a short-circuit between the cable l0 and 4 its shield I lresults in a similar intermittent current across resistance 21.

The circuit as shown in Pig. 2 has two independent adjustments for thesensitivity of tube 46. For example, the initial bias can be adjusted bymeans of battery 46, and the potential applied by load resistance 81 canbe adjusted by moving tap ll.

Referring back to Fig. L-it will be seen that the sensitivity of theupper branch of the circuit. comprising the upper pair of cathodefollower tubes 6, connected to resistance It, can be adjustcd by tap II,and the sensitivity of the lower branch, comprising the pair of cathodefollower tubes 6 connected to resistance i6. can be independentlyadjusted by tap II.

From the description of operation with respect to Fig. 2, it will beapparent, therefore. that in Fig. 1 any defect or failure in one of thedetecting branches results only in loss of signal from the defectivebranch. The operation of the remainder of the detecting elements beingunaffected. the strength of the signal applied to grid 42 for a givenlight by any one of the four lower phototubes depends upon theadjustment of tap ll, while the strength of the signal applied to thegrid tube 42 for a given light intensity of any one of the 4 upperphototubes I may be adjusted independently by tap ii. The circuit shown,therefore, has two independently adjustable sensitivity levels. Anydesired number of detecting branches may be added in each sensitivitylevel, and any desired number of sensitivity levels may be added. byconnections similar to those shown. Regardless of the number ofdetecting channels used in the system, no current will flow through tube46 unless one or more of the phototubes is illuminated and itsassociated circuit is in proper operating condition.

In the circuit of Fig. l. adjustment of the initial negative bias ontube 46 is provided .by tap 36 on the power supply output resistance 46.A normally closed push-button ll shunts out part of resistance 64. Ifthe push-button is temporarily depressed, the resistance of the powersupply output circuit is increased and a decrease in output currentresults. The bias applied by tap it to grid 42 is; therefore, decreased.In setting one of the taps l8, is to compensate for the ambient lightlevel at the phototubes, the push bution 36 is depressed and the tap it(or i!) adjusted until signal light 44 just goes out. When the pushbutton 36 is released, a fixed amount of additional negative bias isapplied to grid 42, so as to give a predetermined safety margin whichmust be overcome before the tube 46 will respond to increased excitationof one of the phototubes. Reliability of the system is thus greatlyincreased by eliminating false alarms due to slight fluctuations inambient light at the phototubes.

Reliability of the system regardless of appreciable line voltagefluctuations is insured by the stabilizing eifect of tap 26. If the linevoltage increases, the output current of the power supply increases andthe corresponding increase in the negative bias applied by tap 26 togrid 42 compensates for the rise in plate voltage across tube 40 andprevents the tube from conducting.

From the foregoing description, it is apparent that a device constructedaccording to this invention provides substantially instantaneousdetection of fire, and is reliable and stable to a high degree undersevere operating conditions. The system contains no moving parts and sois not subject to vibration eflects. Since a relatively small number ofelectrical components, themselves negligible in size and weight, isadequate for supervision of a large space, the device is ideally suitedfor use on aircraft or other vehicles. The fact that the systemcontinues to operate as long as one detecting channel remains intact isof crucial importance in aircraft, as the system will continue toprovide a considerable measure of fire protection in spite of severedamage from engine failure, previous fire, enemy action, or othercauses.

Since certain changes may be made in the above described article anddifferent embodiments of the invention could be made without departingfrom the scope thereof, it is intended that all matter contained in theabove description or shown in the accompanying drawings shall beinterpreted as illustrative only and not in a limiting sense.

What is claimed:

1. A fire detection device comprising: a plurality of detectingbranches, each of which comprises an electron discharge device having ananode, cathode, and control electrode, a direct current source connectedbetween said anode and cathode, a capacitance connected between saidcathode and control electrode, and connected in series with saidcapacitance, a first alternating current source and a radiationsensitive element adapted to pass a rectified current when exposed toradiation; an electronic amplifier tube having input and outputterminals; a common load resistance connected in the anode-to-cathodecircuits of all said discharge devices and also connected to the inputterminals of said amplifier tube; and, connected between said outputterminals, an electrical signalling device and a second alternatingcurrent source, the phase of which is substantially opposite to that ofsaid first aiternating current source.

2. A device according to claim 1 wherein said direct current sourcecomprises, in series connection, an alternating current; source, arectifier tube, and means for applying an initial negative bias to saidinput terminals.

3. A device according to claim 1 wherein said direct current sourcecomprises, in series connection, an alternating current source, arectifier tube, means for applying an initial negative bias to saidinput terminals, and a resistance shunted by a normally closed switch.

4. A device according to claim 1, each of said detecting branchesincluding a cathode follower resistance in its cathode circuit.

5. A device according to claim 4 having means for independently varyingthe magnitude of each of said cathode follower resistances.

' DONALD J. MACDOUGALL. PHILLIP J. CADE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

